This invention relates to accurate measurement of voltage, current and phase of RF power applied to a non-linear load, and is particularly directed to probes for detecting the current, voltage, and phase of radio frequency (RF) electrical power that is being supplied to an RF plasma chamber.
In a typical RF plasma generator arrangement, a high power RF source produces an RF wave at a preset frequency, i.e., 13.56 MHz, and this is furnished along a power conduit to a plasma chamber. Because there is typically a severe impedance mismatch between the RF power source and the plasma chamber, an impedance matching network is interposed between the two. There are non-linearities in the plasma chamber, and because of these and because of losses in the line and in the impedance matching network, not all of the output power of the RF generator reaches the plasma chamber. Therefore, it is conventional to employ a probe in close proximity to the power input to the plasma chamber to detect the voltage and current of the RF wave as it enters the plasma chamber. By accurately measuring the voltage and current as close to the chamber as possible, the user of the plasma process can obtain a better indication of the quality of the plasma. This in turn yields better control of the etching or deposition characteristics for a silicon wafer or other workpiece in the chamber.
At present, diode detection probes are often employed to detect the amplitude of the current and voltage waveforms. These probes simply employ diode detector circuits to rectify the voltage and current waveforms, and deliver simple DC metering outputs for voltage and for current. These probes have at least two drawbacks in this role. Diode detectors are inherently non-linear for low-signal levels, and are notorious for temperature drift. The diode detector circuits also are limited to detecting the signal peaks for the fundamental frequency only, and cannot yield any information about higher or lower frequencies present in the RF power waveform. In addition to this, it is impossible to obtain phase angle information between the current and voltage waveforms, which renders the power measurement less accurate.
One proposal that has been considered to improve the detection of RF power has been to obtain digital samples of the voltage and current outputs of a probe, using flash conversion, and then to process the samples on a high-speed buffer RAM. However, this proposal does have problems with accuracy and precision. At the present time, flash conversion has a low dynamic range, normally being limited to eight bits of resolution. To gain reasonable phase accuracy for plasma customer requirements, it is necessary to reach a precision of at least twelve bits, so that a phase angle precision of better than one degree can be obtained at full power. In addition, flash converters require an extremely fast RAM in order to buffer a block of samples before they are processed in a digital signal processor (DSP), and fast RAM circuitry is both space-consuming and expensive.
Voltage and current probes that are now in existence are limited in their performance by the fact that they can only monitor the voltage, current, and phase angle at one frequency, and even then such probes have a poor dynamic range. Examining behavior at a different frequency requires changing out the hardware, which can be costly and time consuming. This means also that good performance will ensue only if the load is linear, which is never the case with a plasma chamber. Unlike capacitors, inductors, and resistors, plasma chambers impose a highly non-linear load, which causes the sinusoidal waveform of the input power to become distorted. This distortion causes the resulting waveform to be a sum of sinusoids, with the frequency of each additional sinusoid being some integer multiple of the input sinusoidal frequency (i.e., harmonics). Conventional probes can provide voltage, current and coarse phase information, at best, for the fundamental voltage and current waveforms. This severely limits the accuracy of the system, and makes accurate and repeatable control impossible when there is a significant amount of voltage or current appearing in the harmonics.
A possible solution to this has been proposed in U.S. patent application Ser. No. 08/684,833, now U.S. Pat. No. 4,770,922, and having the same assignee as the present patent application. In that case, the voltage/current probe employs a frequency shifting arrangement that converts the sampled voltage and current to a lower frequency baseband signal to facilitate accurate detection of RF current and voltage of the applied power, as well as phase information, with the baseband being approximately 0.2 KHz to 15 KHz. The baseband voltage and current signals are digitized and processed to obtain voltage and current information, and using complex fast fourier transform technique, to obtain accurate phase information. That application Ser. No. 08/684,833 is incorporated herein by reference.
Even with this technique, it remains to provide a super-high matching directivity voltage and current sensor that behaves as if it has a zero probe length, and which accurately reports the voltage, current, and phase conditions at the RF load. The problem in doing this arises because any real voltage probe and any real current probe will have a finite length, and the current and voltage waveforms are not flat over the length of the sensor.
A voltage-current probe, or V/I probe, is a sensor that is intended to produce output signals that represent a zero-length point at which it is inserted. On the other hand, any realistic sensor must be of a finite size to sense the voltage or current. The V/I probe produces a low-level signal which has a well-defined relationship with respect to the high-level signal (i.e., applied current or applied voltage) that is being measured. The fact that the probe or sensor has finite length, coupled with the fact that the applied power and the real-world non-ideal load produce standing waves, means that the RF voltage (or current) is not going to be identical everywhere along its finite length. It is also the case that the effect of non-uniformity over the length of the sensor increases at higher frequencies, e.g., harmonics of the applied RF power. Unfortunately, nothing in the current state of the art compensates for this, and a calibration algorithm for the V/I probe is heretofore unknown in the art.
It is an objective of this invention to provide a reliable and accurate probe, at low cost, for detecting the current and voltage of RF power being applied to a plasma chamber and for accurately finding the load impedance (which may have real and imaginary components) as well as phase angle between the voltage and current applied to the load.
It is a more specific object of this invention to provide an improved voltage and current pickup head that accurately measures the RF voltage and current at the point of injection of an RF power wave into an RF load.
It is a further object to provide a V/I probe with a calibration algorithm to compensate for the non-zero length of the voltage and current sensors of the probe.
According to an aspect of the invention, RF voltage and current levels and relative phase information for current and voltage can be derived for an RF power wave that is applied at a predetermined RF frequency to a load, such as the power input of a plasma chamber. The V/I probe produces a voltage pickup value VV and a current pickup value VI. However, because the sensors for voltage and current are of finite length, and are not simply points, the technique of this invention compensates to produce corrected values of voltage, current, as well as impedance and phase. This involves computing the voltage as a complex function of the voltage pickup signal and the current pickup signal, based on coefficients precalibrated for the particular operating radio frequency, and also computing the current as a complex function of the voltage and current pickup signals based on coefficients pre-calibrated for that operating radio frequency. This can also be carried out by computing a corrected voltage value as a complex function of the voltage pickup signal and the current pickup signal, based on coefficients precalibrated for the operating radio frequency, computing a complex impedance of the load at the operating radio frequency on the basis of the voltage and current pickup signals, and then computing a corrected current value based on the corrected voltage value and the complex impedance. By xe2x80x9ccomplex impedancexe2x80x9d it should be understood that the load impedance may have both a xe2x80x9crealxe2x80x9d or resistive component and an xe2x80x9cimaginaryxe2x80x9d or reactive (either capacitive or inductive) component.
The signal processor is calibrated with correction factors for each of a number of frequencies within a range. These values are acquired by obtaining voltage and current pickup signals, for each of a number of calibrating radio frequencies within the range, under conditions of (a) open circuit load, (b) short circuit load, (c) fixed known impedance load, e.g., fifty ohms, and (d) either a known calibration voltage or a known current being applied at a precise input level to a known load from an RF calibration source. Then the digital signal processor computes and stores the correction coefficients based on the voltage and current signal values obtained under these conditions (a) to (d). These coefficients are obtained for each of the calibration frequencies in the overall range of interest. When the system is operated at a particular selected operating frequency, the stored correction coefficients are applied to the voltage and current pickup signals to obtain corrected voltage, corrected current, and a corrected load impedance value.
Computing the amplitudes and relative phase of the voltage and current signals is carried out in the digital signal processor. For any operating frequency between successive calibrating radio frequencies for which the correction coefficients are stored, correction coefficients are applied by interpolating between stored values for each such correction coefficient for the calibration frequencies above and below the selected operating frequency.
The voltage and current probe has a metal housing having a cylindrical bore therethrough, and first and second recesses, the recesses each opening to said bore for an axial distance. Voltage and current sensor boards fit into these recesses, as discussed shortly. There is a center conductor extending along the axis of the bore and a cylindrical insulator within the bore. The insulator surrounds the center conductor and extends radially between the conductor and the housing. A voltage sensor board is mounted in the first recess and has a capacitive pickup plate facing radially towards the axis of the bore. The current sensor board is mounted in the second recess and has an elongated inductive pickup conductor, e.g., a wire, positioned to face radially towards the the of the bore and extending axially for the predetermined distance mentioned earlier. In a preferred embodiment, the first and second recesses are positioned opposite one another on the metal housing across the axis of the bore.
In many possible embodiments the voltage sensor board and current sensor board each can include, in order radially outward, the capacitive plate or pickup wire, an insulator layer, a ground plate completion conductive layer, and a circuit board carrying voltage pickup components, with at least one electrical conductor passing from the capacitive plate or inductive wire through openings in the insulator layer and the ground plate completion conductive layer to said respective voltage or current pickup components.
In similar fashion, the current sensor board can be formed, in order radially outward, of the inductive pickup conductor, an insulator layer, a ground plate completion conductive layer, and a circuit board carrying current pickup components. At least one electrical conductor passes from each end of the inductive pickup conductor through openings in the insulator layer and ground plate completion conductive layer to the current pickup components. Preferably the voltage and current pickup components are symmetrically distributed upon the respective boards, both geometrically and electrically.
The above and many other embodiments of this invention will become apparent from the ensuing detailed description of a preferred example, which is to be read in conjunction with the accompanying Drawing.